1. Field of the Invention
The present invention relates generally to a scroll compressor having stationary and orbiting scrolls in engagement with each other and, more particularly, to tip seal grooves formed in spiral wrap elements of the stationary and orbiting scrolls.
2. Description of Related Art
Generally available scroll compressors comprise a stationary scroll and an orbiting scroll, both of which have respective wrap elements each having an internal wall and an external wall opposite to each other.
In almost all of the scroll compressors, the internal and external walls are shaped so as to represent two involute curves, both of which are based on an identical involute base circle, but have different phases and, hence, the wall thickness thereof is made constant from an inner end to an outer end of each of the wrap elements. The compressing action is achieved by circling one of the wrap elements relative to the other under the condition in which the two wrap elements are circumferentially shifted 180.degree. one from the other with the side walls thereof held in partial contact with each other.
Each of the wrap elements has a tip seal accommodated in a groove defined therein on an axial outer end thereof. The tip seal is held in sliding contact with an end surface of the confronting wrap element to establish an axial seal. The tip seal groove has opposing internal and external walls that are formed so as to represent two involute curves having different phases, like each of the wrap elements.
The internal wall of one of the two wrap elements can be shaped to represent a curve developing from the center of a base circle so as to extend spirally outwardly therefrom, while the external wall of said one of the two wrap elements is shaped based on the shape of the internal wall. In this case, the other wrap element is shaped with envelope curves obtained as a result of circular translation of said one wrap element relative thereto. The scroll compressors of this type have the advantage of being capable of enlarging the volume of entrapment, if the cylinder bore is identical, by making the factor of development large the factor of development being normally referred to as the constant (a) in the equations f.sub.1 (.o slashed.)=a.o slashed., and f.sub.0 (.o slashed.)=a.o slashed.+b, compared with those of the aforementioned type having the wrap elements so shaped as to represent the involute curves. If the intake volume is identical, the outer diameter of a shell can be reduced, contributing to the compactness of the whole compressor as to the compressor being lightweight.
However, in the scroll compressors having the wrap elements so shaped as to represent such curves, if the internal and external walls of the tip seal groove are shaped so as to represent the same curves as those of the wrap elements, the groove width gradually increases in a direction towards the end of winding i.e., the outer end of the tip seal groove. Because the tip seal is of an identical cross section over the length thereof, the clearance between the tip seal, which is usually held in contact with the internal wall of the tip seal groove, and the external wall of the tip seal groove varies in a direction longitudinally thereof, thus causing a problem associated with sealing properties.
Also, in machining the tip seal groove, although a groove having an identical width can be formed through a single end milling operation, at least two milling operations are required for that having a varying width. In addition, the groove having a varying width is extremely inferior in processability to that having an identical width.